Cylindrical and conical filter elements used in filter assemblies for filtering fluids are frequently configured with a cylindrical or conical media in which fluid being filtered passes radially through the media either from, or to, a hollow central core which is plugged at one end. In order to appropriately mount the filter media, the filter media is frequently provided with a radially projecting, annular gasket for sealing the media with respect to inlet and outlet openings.
A conventional practice in forming the gasket is to expand within a mold cavity foamable material such as polyurethane produced by a mixture of polyol and isocyanate. When using this process, the expanding material produces flash between upper and lower mold halves. The flash must be trimmed and vacuumed off prior to sale of the filter element; otherwise, the flash may become dislodged during installation or operation. If the flash is sucked against the filter media, the media's efficiency is reduced. There is also the chance of loose flash being transferred into the engine or other machine on which the filter employing the media is installed and meant to protect.
Another difficulty which arises when forming such annular seals is the occurrence of pits or other discontinuities in the surface of the gasket due to air or other gas bubbles being trapped as the material from which the gasket is formed expands into engagement with the surface of the top mold. In addition to presenting an unfinished appearance, these defects can on occasion result in a leaking seal which can allow at least small debris to be sucked around the gasket into the engine. In addition, the strength of the gasket may be compromised if large air bubbles are trapped in the foam material as it expands.